MIL-HDBK-1025/5
coastal site, it is possible to interpolate predictions for the site from
values given in the National Oceanic and Atmospheric Administration Tide
Prediction Tables for the two nearest stations.
Extremes of the predicted spring tides provide criteria for
b)
small craft harbor design accommodating any water level fluctuations that may
occur.
Water Area Shoaling Factors
2.2.3
Littoral Drift
2.2.3.1
The principal cause of shoaling at entrances to harbors is
a)
littoral drift.
The longshore movement of sand is due mainly to wave action.
b)
Structures that change the normal regimen of waves breaking along a coast may
influence the littoral movement.
If the approach of the prevailing waves is normal to the
c)
shore, the initial effect is movement of the littoral material from the lips
inward along each flank of the channel, thereby eroding the lips and shoaling
the inner channel. As the process continues, the channel banks accrete toward
the center of the channel, fed by material from the beach on either side of
the entrance.
Where prevailing wave approach is oblique to the shoreline,
d)
sediments being transported along the shore by littoral currents will be
interrupted at the channel opening near the up drift lip and that lip will
soon begin to accrete. As the wave-induced longshore current again begins to
impinge on the shore down drift of the channel mouth, it attempts to reacquire
its sediment load. As a result, the down drift lip of the channel will erode
at about the same rate as the up drift lip accretes, and the channel mouth
will migrate in the down drift direction.
To minimize entrance shoaling, install jetties along each
e)
flank of the entrance channel from the lips of the mouth seaward beyond the
breaker zone. Structural features of jetty construction shall prevent the
materials from washing through or over the structure into the channel.
River Discharge
2.2.3.2
Harbors in off-river basins may undergo shoaling due to
a)
sediment deposition in the quiet-water area and eddy currents that may be
created by the entrance configuration and the flowing water in the river.
To minimize shoaling, provide a flat area on the downstream
b)
lip of the entrance from which a dragline can excavate deposits from the
bottom of the entrance channel, and cast the deposits into the river
downstream from the entrance. A permit from the State Marine Resources
Commission and the Corps of Engineers is required for water disposal of the
excavated deposits. The entrance shall be maintained narrow to permit such an
A training dike installed off the upstream will reduce sediment
operation.
(See Figure 2.)
deposition.
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